In law, a reasonable person, reasonable man, or the man on the Clapham omnibus, is a hypothetical person of legal fiction crafted by the courts and communicated through case law and jury instructions.

Strictly according to the fiction, it is misconceived for a party to seek evidence from actual people to establish how the reasonable man would have acted or what he would have foreseen. This person's character and care conduct under any common set of facts, is decided through reasoning of good practice or policy—or "learned" permitting there is a compelling consensus of public opinion—by high courts.

In some practices, for circumstances arising from an uncommon set of facts, this person is seen to represent a composite of a relevant community's judgement as to how a typical member of said community should behave in situations that might pose a threat of harm (through action or inaction) to the public. However, cases resulting in judgment notwithstanding verdict can be examples where a vetted jury's composite judgment was deemed beyond that of the reasonable person, and thus overruled.

The reasonable person belongs to a family of hypothetical figures in law including: the "right-thinking member of society", the "officious bystander", the "reasonable parent", the "reasonable landlord", the "fair-minded and informed observer", the "person having ordinary skill in the art" in patent law, and stretching back to Roman jurists, the figure of the bonus pater familias, all used to define legal standards. While there is a loose consensus in black letter law, there is no accepted technical definition. As with legal fiction in general, it is somewhat susceptible to ad hoc manipulation or transformation, and hence the "reasonable person" is an emergent concept of common law. The "reasonable person" is used as a tool to standardize, teach law students, or explain the law to a jury.

As a legal fiction, the "reasonable person" is not an average person or a typical person, leading to great difficulties in applying the concept in some criminal cases, especially in regard to the partial defence of provocation. The standard also holds that each person owes a duty to behave as a reasonable person would under the same or similar circumstances. While the specific circumstances of each case will require varying kinds of conduct and degrees of care, the reasonable person standard undergoes no variation itself. The "reasonable person" construct can be found applied in many areas of the law. The standard performs a crucial role in determining negligence in both criminal law—that is, criminal negligence—and tort law.

The standard is also used in contract law, to determine contractual intent, or (when there is a duty of care) whether there has been a breach of the standard of care. The intent of a party can be determined by examining the understanding of a reasonable person, after consideration is given to all relevant circumstances of the case including the negotiations, any practices the parties have established between themselves, usages and any subsequent conduct of the parties.

The standard does not exist independently of other circumstances within a case that could affect an individual's judgement.